Polytetrafluoroethylene (PTFE) is the common designation given to fully fluorinated polymers which have the basic chemical formula (--CF.sub.2 --CF.sub.2 --).sub.n and that contain 76 percent by weight fluorine. PTFE polymers are opaque solids having a white color and a crystalline melting point of about 327.degree. C. PTFE is formed from tetrafluoroethylene monomer, CF.sub.2 =CF.sub.2 which is a low boiling organic liquid (boiling point -76.3.degree. C. at 760 mm) which exists in the gas phase under normal polymerization conditions. As is well known to those familiar with PTFE, tetrafluoroethylene polymerizes readily at moderate temperatures and pressures (e.g. 60.degree. C. and 5-25 atm) in the presence of free radical initiators.
As is further known by those familiar with PTFE materials, and to a great extent by the general public, PTFE possesses an outstanding combination of chemical and physical properties including excellent chemical resistance, stability at high temperatures, superior dielectric properties, good anti-frictional and "non-stick" properties and excellent resistance to degradation under typical outdoor weathering conditions.
A thorough discussion of polytetrafluoroethylene, its manufacture, fundamental properties, its fabrication into end products, and the respective properties of such products, is given in S. Sherratt "Polytetrafluoroethylene" (1966), Encyclopedia of Chemical Technology, Volume 9 (John Wiley & Sons, Inc.), the contents which are incorporated entirely herein by reference.
One of the desirable properties for certain end products of PTFE used in particular applications, is porosity. Porous PTFE products often have deformable and flexible properties which make them especially useful as a chemically inert sealing material when positioned between various mechanical parts. In general terms, PTFE is an excellent material for forming gaskets, pipe thread seals, and the like.
As is the case with many organic polymers, however, PTFE is not normally a porous material. Accordingly, a number of techniques have been devised for producing porous PTFE. One type of process for fabricating porous PTFE comprises adding a filler to the PTFE prior to forming the desired article and then removing the filler from the PTFE after the article is formed. The technique accordingly leaves behind void spaces which in turn give the end product the desired porosity. Typical fillers are leached out with solvents or melted out at elevated temperatures. In some cases polymers are used as fillers which are depolymerized and removed to produce the porous product. Such techniques can be somewhat complicated, however, and in cases where the solvent is not entirely removed, serious flaws can result in the end product.
More recently, and as set forth in U.S. Pat. Nos. 3,953,566 and 4,187,390 to Robert W. Gore (the "'566 patent" and the "'390 patent", respectively), a method of making porous PTFE can comprise a particular sequence of extruding, stretching, and then heating PTFE to form the desired product.
In this regard, the nature of PTFE is such that many normal techniques for manufacturing and handling polymers are inappropriate for manufacturing and handling PTFE. In particular, PTFE does not flow in the melt above its crystalline melting point of 327.degree. C. As a result, typical melt flow techniques will not work with PTFE. Accordingly, PTFE must be processed in methods that are generally unconventional for polymers, but more closely resemble the techniques of powder metallurgy.
In a typical technique, PTFE is provided in the form of a "coagulated dispersion" which is then extruded to a desired shape and then sintered--i.e. heated somewhat above its melting point for a time sufficient for it to coalesce into an essentially impermeable material--to produce the resulting product. A thorough discussion of such processing techniques is set forth in "The Processing of PTFE Coagulated Dispersion Powders", pages 1-36, which is available from ICI Americas, Inc., Wilmington, DE 19897, and the contents of which are also entirely incorporated herein by references. Other handling techniques are set forth in "Health and Safety Aspects of Fluon Polytetrafluoroethylene", Technical Service Note F10, Fourth Edition, copyright Imperial Chemical Industries PLC 1985, which is also available from ICI Americas, Inc., Wilmington, DE 19897, and which is likewise incorporated entirely herein by reference.
As discussed in these references, PTFE coagulated dispersion ("CD") polymers are typically supplied as a fine, free flowing powder having an average bulk density of about 500 grams per liter and an average agglomerate size of between 450 and 500 microns. These dispersions are manufactured by coagulating an aqueous dispersion of PTFE. In order to extrude such dispersions, the coagulated dispersion polymers are blended with an "extrusion aid" or lubricant. A typical lubricant is a hydrocarbon having a desired vaporization temperature; examples are petroleum ether, naphtha, and low odor paraffin solvents. A number of such solvents, and the reasons for selecting a particular solvent are also discussed in the various references incorporated above. The general criteria for selecting a lubricant are well known and an appropriate lubricant for a particular manufacturing technique or end product can generally be selected without undue experimentation. Typically, lubricant is added in a proportion of about 15 to 25 percent of the total weight of the composition.
The dry-appearing mix of dispersion and lubricant is then lightly pressed into a billet or preform. Such preforms are stiff and brittle and similar in consistency to soft wax candles. The preform billet is then forced through the simple die of a constant rate ram extruder. The extrudate typically passes into a drying oven, the temperature of which is sufficient to vaporize the lubricant, and if desired, the extrudate is passed through another oven where it is sintered.
During extrusion the PTFE forms oriented fibers. These give the extrudate considerable longitudinal strength, but virtually no transverse strength.
The properties of the extrudate that results from these techniques will depend upon the combined effect of a number of variables. The most important of these include: the type of CD polymer; the reduction ratio; lubricant type and content; die cone angle; die parallel length; die temperature; and extrusion rate. Of these, the reduction ratio largely determines the amount of work done on the polymer, and in turn most significantly affects the properties of the extrudate. Reduction ratio can be defined in a number of ways, the simplest of which is the ratio of the cross sectional area of the extrusion cylinder to that of the die. The hardness or deformability of the extrudate is largely controlled by the work applied to the polymer during extrusion, and reduction ratio is a measure of such work. The more the polymer is worked the more fibrillated and harder it becomes, therefore, control of the reduction ratio will to a significant extent control the properties of the resulting product.
In the Gore '390 patent, porous PTFE is produced by extruding a paste resin at relatively high reduction ratios of at least 100, and usually much greater. The extrudate is then stretched at a relatively rapid rate, i.e. on the order of 100 percent per second or more. Subsequent to stretching, the stretched extrudate is sintered at temperatures above the crystalline melting point. The resulting product has a particular microscopic structure and a matrix tensile strength in at least one direction above about 7300 pounds per square inch (psi). The '390 and '566 patents, however, disclose that the increase in strength of a polymer matrix following sintering depends upon the strength of the extruded material prior to expansion (e.g. column 4, lines 32-36 of the '390 patent). Accordingly, the extrudates are worked at relatively high reduction ratios, (Example 1 discusses ratios of 370) to give the more fibrillated extrudate.
Although higher matrix strengths in porous PTFE products are advantageous in certain circumstances, lower strengths are advantageous in other circumstances. For example, porous PTFE products with lower matrix strengths are deformable under less force. As a result, gaskets made from such material can be appropriately tightened in place with less tightening force while still providing an excellent seal. Lower tightening forces can be advantageous in circumstances where the materials being joined are relatively fragile. For example, many pipes used for carrying various corrosive chemicals are formed of fiberglass or fiberglass reinforced plastic ("FRP"). When threaded or flanged together, such pipes cannot withstand the same tightening pressures that steel or galvanized iron pipes can withstand. In such circumstances, gasket material that is appropriately flexible under less force is desirable.
In forming porous PTFE products by carrying out the procedures in the '390 and '566 patents, both the rate of expansion and the temperature at which expansion takes place have to be controlled in a particularly sophisticated manner. Thus, in these techniques if a particular percent stretch is desired; e.g. 200 percent stretch; the strength and porosity of the resulting product will depend upon the rate at which the stretch takes place; e.g. 30% per second to 500% per second. Thus, the Gore techniques call for stretching the unsintered extrudate at a rate exceeding about 10% per second while the temperature is maintained below the crystalline melting point of the PTFE during the stretching step; i.e. the Gore techniques require that the extruded PTFE be stretched at temperatures below 327.degree. C. More particularly, it has been more recently determined that a more characteristic description of the Gore technique is a rate of stretch of about 100% per second or higher. As further determined, the term "stretch rate" as used in the Gore patent and by those skilled in the art, refers to the percent of stretch divided by the time of stretching.
In manufacturing techniques, however, the careful control of such stretch rates and temperatures require the careful control of several different variables. Accordingly, if the process could be simplified somewhat, it would offer significant advantages in cost and technique. Additionally, as stated above, the high matrix tensile strength of the products in the '390 patent produced by the '566 method are not always satisfactory for applications in which a lower matrix tensile strength is desired For example, a lower matrix tensile strength, as well as a higher density, improves the cold flow or "creep" properties of porous PTFE in addition to the other advantages of lower matrix tensile strength set forth previously.